1. Field of Invention
This invention relates to integrating a microphone into an integrated circuit made using standard processing.
2. Description of Prior Art
In systems using microphones, the means of converting the sound energy into electric energy involves some sort of material which flexes when the sound wave strikes, and then this flexing is converted into electrical energy.
In many electronic systems where size is very limited such as cell phones and hands-free telephones such as speakerphone® telephones, the microphone is a “condenser” microphone. The equivalent circuit of a condenser microphone is a capacitor, C, typically on the order of 0.15 uF, and a resistor, typically on the order of 3 kΩ, in series. A bias voltage, Vb, is applied, and charge, Q, is stored on the capacitance according to the well known definition of a capacitor, Q=C×Vb. The sound pressure causes the capacitor to change value, and the physics of this operation are well known to those skilled in the art. Since the capacitor value, C, changes, but the voltage, V, is fixed, the charge, Q, must change. Current is defined as the change in charge per unit time, so changing the sound pressure results in a current. This current is applied to a low noise amplifier capable of increasing the signal to a level appropriate for the application. Since the condenser microphone is a separate element from the rest of the electronics, its piece-part cost and assembly space and cost add to the system cost.
Hearing aids are another application requiring a sound-pressure to electrical transducer.
Numerous U.S. patents exist for various means of making the required small transducers. Many, however, relate to means which are not easily implemented in a single integrated circuit along with other electronics: U.S. Pat. Nos. 6,615,667, 6,473,511, 6,343,514, 6,324,907, 6,283,915, 6,249,075, 6,001,666, 5,881,159, and 5,740,258.
U.S. Pat. Nos. 4,610,062 and 4,922,471 involve using a silicon substrate. However, the first patent uses a sacrificial material with deposited film requiring wet integrated circuit processing and introducing materials that are potential ionic contaminants. It is therefore unacceptable for modern processing. The second patent actually requires two such substrates, with a membrane separation, and additional process steps for non-standard etching significantly increasing cost.
U.S. Pat. Nos. 7,146,016, 6,667,189 and 4,533,795 disclose inventions enabling the microphone to be incorporated directly into a standard integrated circuit process, however, they require considerable additional processing on the active surface as well as back etching. This additional processing is not available as part of any low cost integrated circuit manufacturing operation.
U.S. Pat. Nos. 6,426,239 and 6,156,585 also disclose inventions enabling the microphone to be incorporated directly into a standard integrated circuit process, however, they also require considerable additional processing for back etching.
None of the prior art describes means for using the techniques in differential structures, and such structures are essential for good quality noise performance when integrating analog signal processing circuits on integrated circuits containing the kinds of digital circuitry commonly found in modern electronic systems. Furthermore, unless a differential structure is used, placing active circuitry under the integrated microphone would result in intolerable degradation in performance.